The analysis of gaseous samples for various air pollutants particularly NO and NO.sub.x, has become increasingly important in recent years. One analytical technique which has proved particularly useful is the chemiluminescent reaction between NO and ozone. The quantity of light given off by the reaction is directly related to the quantity of NO in the sample being tested. NO.sub.x must first be converted to NO in a suitable converter for the subsequent chemiluminescent reaction with ozone. The conversion of NO.sub.x to NO is carried out in the presence of a suitable catalyst, such as platinum or activated carbon, at an elevated temperature, normally between 300.degree. C. and 500.degree. C.
The applications for chemiluminescent reactions in the detection of pollutants in gaseous samples was substantially enhanced by the introduction of an atmospheric pressure operated detector as described in U.S. Pat. No. 3,652,227, Neti et al. The design of the apparatus described therein, which forms the basis for many commercially available atmospheric pressure detectors, consists of a pair of nozzles bringing in two directed gas streams to form a point source in an extremely small reaction chamber having a volume on the order of a few cc. A sensitive photo-multiplier tube detector is closely coupled to the reaction chamber. The light emitted due to the reaction between ozone and a reactant gas is electronically measured by the photo-multiplier tube and the associated electronics. As mentioned above in order to measure the NO.sub.x concentration in a sample, the NO.sub.x must be converted to NO prior to its reaction with the ozone in the reaction chamber. This was accomplished by passing the sample gas containing NO.sub.x through a converter having a bed of vitreous carbon maintained at a temperature of between 300.degree. C. to 500.degree. C., see Neti et al. U.S. Pat. No. 4,081,247. In view of the relatively high temperatures at which the NO.sub.x is converted to NO and the resultant high reaction temperatures, the converter housing and the reaction cell must be formed of a material that is non-reactive with the sample gases at the reaction temperatures. The converter housing is conventionally formed from quartz tubing and the reaction chambers for atmospheric pressure chemiluminescent detectors are normally machined from teflon. In order to achieve maximum signal strength and speed of response, the internal volume of the reaction chamber should be held to close tolerances and to a minimum volume. In addition the sample gas stream and the ozone are brought together at a precise angle in order to form a point source.
Instruments built to these specifications are highly satisfactory for their purpose but the expense of manufacturing such instruments makes them unaffordable for low cost, high volume applications such as automobile emission testing. Accordingly it would be highly desirable to provide a low cost chemiluminescent detector operable at atmospheric pressure which has the sensitivity and response speed of high quality, high cost chemiluminescent detectors presently available.